1. Field of the Invention
The present invention relates to integrated-circuit package designs and, more specifically, to improved molded-plastic package designs for thermally enhanced integrated circuits.
2. Prior Art
FIG. 1A shows a conventional quad flat package (QFP) assembly 10 for an integrated-circuit die 12. The integrated-circuit die 12 is attached to an upset, or offset, die-attach paddle portion 14, which is at the center region of a conventional lead frame 16. Various inwardly-extending leads terminate at their inner ends in bonding fingers (typically shown as 18 and 20 ). The bonding fingers 18 and 20 are connected to respective bonding pads on the integrated-circuit die 12 using respective bonding wires (typically shown as 22), as indicated in the Figure. The entire assembly described above is conventionally encapsulated in a molded plastic material, which forms a molded-plastic body 26 for the package assembly 10.
FIG. 1B shows a plan view of the lead frame 16 for the conventional quad flat package (QFP) assembly 10. Note that the ends of the bonding fingers do not extend all the way to the die-attach paddle and are not directly connected to the die-attach paddle 14. This provides substantial spaces 30, 32, 34, 36 between the inner ends of the bonding fingers 18 and 20 and the die-attach paddle 14 of FIG. 1A for the flow of molding compound during an encapsulation process. The assembled die 12 and lead frame 16 combination are encapsulated in molded plastic material by being placed in a cavity formed by the two halves of a mold and by having plastic material be injected into the top half of the mold at one corner of the lead frame. Air vents are provided in the mold at the other three corners of the package. Some of the paths for the plastic material to flow into the bottom half of the mold from the too half of the mold are provided by the spaces 30, 32, 34, which are provided between the inner ends of the bonding fingers and the die-attach paddle. Other paths are provided by the spaces between the leads of the lead frame. These flow paths permit the flow of the plastic molding material to be substantially balanced between the top half and the bottom half of the of the mold as the plastic material flows through the mold. As plastic material is conventionally injected into the mold, air is expelled out of the air vents at the three comers of the mold by the flowing plastic material so that no air remains trapped within the molded-plastic body. If air were to be trapped, it would cause voids, blow holes, or pin holes, in the molded plastic body 26 of FIG. 1A.
FIG. 2 shows a package mold 60 for molding a conventional thermally-enhanced, quad flat package (TE-QFP). The package mold 60 has a top mold-half 62 and a bottom mold-half 64. A thermally-enhanced, electrically-insulated substrate 66, which is formed of a material such as, for example, alumina nitride, has an integrated-circuit die 68 mounted thereto. The thermally conductive, electrically-insulated substrate 66 replaces a conventional die-attach paddle( such as the die-attach paddle 14 of FIGS. 1A and 1B) and improves the thermal performance of a molded-plastic package. Bonding fingers (typically shown as 69 and 70) at the inner ends of the leads of a lead frame 72 are attached to the outer margins of the thermally conductive, electrically-insulated substrate 66 with an adhesive film 73 formed of a polyimide material such as R-flex 1000. As in the case of a conventional quad flat package (QFP) assembly, the thermally conductive, electrically-insulated substrate 66 and its attached integrated-circuit die 68 are placed in the cavity formed between the two halves 62 and 64 of the mold 60. Plastic material is injected into the top half of the mold at the inlet gate 74. The plastic material enters the top half 62 of the mold and flows through the spaces between the bonding fingers of the lead frame into the bottom half of the mold. Vents 76 in the mold comers release trapped air.
The arrows shown in FIG. 2 indicate the flow of plastic molding material through the top half of the mold and through the bottom half of the mold. Note that the thermally conductive, electrically-insulated substrate 66 is much greater in thickness than the conventional die-attach paddle 14 shown in FIGS. 1A and 1B. The intrusion of the much thicker bulk of the thermally conductive, electrically-insulated substrate 66 disrupts and unbalances the flow of plastic material in the mold in several ways.
One way that flow is disrupted is that the open spaces between the ends of the bonding fingers and the edge of the integrated-circuit die are blocked by the substrate 66.
Another way that flow is disrupted is that the bulk of the substrate 66 intrudes into the lower half of the mold so that the cross-sectional area for flow of molding material in the lower space of the cavity is smaller and the flow resistance is greater for the lower space. This causes in the flow of the molding material in the upper half of the mold to be faster than the flow of molding material in the lower half of the mold. As a result of these differences in flow, the air at different places within the mold halves is expelled at different rates so that, for example, some air is trapped within the bottom part of the mold. The trapped air creates voids, also called blow holes or pinholes, in the body of the package. A typical void 80 is created on the side of the package which is opposite the inlet gate 74, as illustrated in FIG. 2.
Consequently, a need exists for a technique to prevent voids on the body of a thermally-enhanced molded plastic package.